1) This has only happened once, I sync all my video/audio in a sequence, start a new 'multicam' sequence, sound works fine but as soon as I click Clip/multi-camera/enable the sound disappears. I HAVE CHECKED that the correct audio stream is selected in my original timeline however the sound wont play?! But if I Un-enable the multicam, the sound comes back

2) This happens a lot. As I'm editing in multi-camera editing after a while and a number of cuts/edits, the sound just stops at that edit onwards. So for instance I'm editing a 10 min piece with 20 plus cuts from one camera to another then at a certain point I make a cut to a camera and the sound stops from that point on.

I too am having the sound problem. The audio just disappears after making several camera switching edits. If you hit "undo" the sound will come back at that point but if you once again try to make a camera switch, the sound will be gone again at that edit point forward. It doesn't matter which camera you select after that...the audio will be gone. This has happened to me on two projects. What I did to work around it was to place the same multicamera sequence on the timeline again following the point where the sound disappears. I then edit the added sequence so that it starts at the point where I lost the sound. You can now edit this sequence and select among cameras and the sound will stay. But this time consuming and I shouldn't have to do this. Is it related to the number of camera edits that you put in a sequence?

But, when I do multicam shoots, the first thing I do, is mute the audio track generated in the multicam sequence because its very rare that any rule will result in the right audio being presented in the switched multicam seq anyway. So, I mute it (or remove it completely) and copy the 4 audio tracks from the original synced seq and paste them into the multicam seq. They wil stay in sync and now you have the ability to mix as you wish from any track.

Although I don't know the cause, I kept playing with it and found a fix for this problem. If you lose the sound after making multiple camera edits do this... Go to "Edit>Preferences>General" and near the bottom is a checkbox for "Render audio when rendering video". Check this box. It is not selected by default. After doing this, render your timeline/sequence by hitting the enter key. The sound will now return at the point where it disappeared and you will be able to continue making camera edits. After doing this, I was able to finish my multicamera edit with sound.